ScriptedImporterAttribute does not register the AutoSelect property correctly.
Either the importer is not selected as default where it should or the possible override is not being found by the GetAvailableImporterTypes method.

See repro project logs:
Open the project and run Tools/BugRepportDraw menu.

Expected:
mp4 shows 3 values (VideoClipImporter, SomeVideoImporter and SomeOtherVideoImporter)
mpp shows 2 values (SomeVideoImporter and SomeOtherVideoImporter)
mp3 shows two values (AudioImporter and SomeOtherVideoImporter)

Actual:
The values are all wrong and even addguest.mpp does not have any importer assigned while two importers are registered to this extension.